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Jackrock

Jackrock logo #21044A caltrop or other object manufactured with one or more rounded or sharpened points, which when placed or thrown present at least one point at such an angle that it is peculiar to and designed for use in puncturing or damaging vehicle tires. Jackrocks are commonly used during labor disputes.
Found on http://www.coaleducation.org/glossary.htm

Jackrock

Jackrock logo #22731A caltrop or other object manufactured with one or more rounded or sharpened points, which when placed or thrown present at least one point at such an angle that it is peculiar to and designed for use in puncturing or damaging vehicle tires. Jackrocks are commonly used during labor disputes.
Found on http://www.rocksandminerals.com/glossary.htm
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